An investigation by Mayors Against Illegal Guns found that one out of every three sellers on Armslist.com had a criminal record.

Mayor Michael Bloomberg and his chief policy adviser, John Feinblatt, released a new Mayors Against Illegal Guns report Thursday about online gun sales.

The report comes almost a year after the mass shooting at Sandy Hook Elementary School in Newtown, Conn., that killed 20 young children and six educators. Bloomberg slammed Congress for its inaction on gun control since then.

These guns could end up in New York despite the state's fairly stringent gun control regulations. Unless a website is based in New York, Feinblatt explained, there is little way to enforce city or state law.

Mayors Against Illegal Guns used private investigators to look into gun sellers on ArmsList.com.They found that one out of every three sellers had a criminal record, and that 29 percent sell a high enough volume of guns that they should be required to register for a federal license to sell firearms.

At their current rate, these dealers would sell 243,800 guns a year.

Mayors Against Illegal Guns also found that legislation closing private sales loopholes would be effective: In states where private sellers are required to conduct background checks, 73 percent did so.

Ultimately, they said, it all comes back to Congress requiring all gun sellers to comply with the federal law mandating background checks for buyers.

"Until Congress acts, we will continue to watch the Internet funnel tens of thousands of guns into the hands of dangerous individuals," Feinblatt said. "And we'll continue to watch the horror as our fellow citizens are gunned down."

The mayor said this report is the last Mayors Against Illegal Guns will issue during his tenure, but he vowed to continue his work pushing for gun control.

"We will keep fighting," he said. "We will keep fighting even harder."